Electronic brake systems, such as so-called “brake by wire” systems, are increasingly being integrated into or replacing conventional hydraulic brake systems of motor vehicles including passenger cars, SUV's and light trucks. Such electronic brake systems are preferable because they reduce the mass of the system, provide greater ability to integrate the system into the vehicle's other electronic circuits and controls, and are suited for use with sophisticated brake control systems.
During depression of the brake pedal by a vehicle operator in a conventional hydraulic braking system, the hydraulic fluid will exert a reaction force back on the brake pedal due to the hydraulic pressure in the brake lines. Since an electronic brake system may not have such hydraulic pressure at the brake pedal, the vehicle operator will not detect any countering force, which in turn can disorient the operator. Accordingly, a typical electronic brake system will include a brake pedal feel simulator to provide a simulation force on the brake pedal. The simulation force provided by the simulator acts opposite the brake pedal force generated by the vehicle operator and helps the operator modulate the system.
The Applicants have discovered a drawback to such brake pedal feel simulators in electronic brake systems which rely on brake pedal force as an element of actuating the wheel brakes. During emergency conditions or failure conditions, the brake pedal feel simulator continues to oppose the depression of the brake pedal by the operator, and hence reduces the amount of force transmitted through the brake pedal. As used herein, emergency conditions are defined as situations where a large amount of braking force is required in a short period of time, and generally including brake pedal forces greater than 200 N. As used herein, failure conditions are defined as a power failure or the failure of brake boosters to supplement the braking force, which also generally include high brake pedal forces greater 200 N. Thus, the simulator opposes the operation at times when high brake forces are needed.
Accordingly, there exists a need to provide a brake pedal feel simulator which automatically adjusts its operation to reduce the simulation force during emergency or failure conditions.